[RFC]: Supporting PIO mode of operation in i2c_msg->flags

[vaibhav.hiremath@linaro.org (Vaibhav Hiremath)]

On Thursday 28 May 2015 07:25 PM, Vaibhav Hiremath wrote:
>
>
> On Monday 25 May 2015 03:17 PM, Vaibhav Hiremath wrote:
>>
>> Hi,
>>
>> This is rather an old issue, for which I have also done custom & hacky
>> solution in the past.
>>
>> Please let me know if there is already an some other method or accepted
>> workaround to this issue. Also, let me know if you have any suggestions
>> on this patch, I would be happy to work on it.
>>
>>
>> Problem Statement:
>> ------------------
>> In certain embedded platforms, the board power on/off is managed by
>> PMIC IC, which in most of the cases is over I2C bus.
>> So during shutdown/reboot you may have to do I2C transactions to access
>> PMIC, which may sleep if I2C bus driver is not functioning in PIO by
>> default.
>>
>> This is unexpected, to sleep after you disable the interrupts.
>>
>> For example,
>>
>> During reboot/shutdown, the execution call reaches to,
>>
>> void machine_power_off(void)
>> {
>>          local_irq_disable();
>>          smp_send_stop();
>>          if (pm_power_off)
>>                  pm_power_off();
>> }
>>
>> pm_power_off will try to do I2C transaction to access PMIC, which may
>> sleep/schedule.
>>
>>
>> Current implementation:
>> --------------------
>>
>> Everyone probably does have their own custom implementation for this.
>> Or some other mechanism to achieve this (may be hardware support).
>>
>>
>> Proposal:
>> ---------
>>
>> If I am not missing any other way of handling this situation,
>> below proposal should be clean and acceptable (I hope so :) )
>>
>>
>> Add flags to choose PIO mode, for I2C transactions, and the default
>> would be to non-PIO mode.
>>
>>
>> struct i2c_msg flags :
>>
>> #define I2C_M_PIO    0x0800
>>
>>
>> And also add respective functionality flags
>>
>> #define I2C_FUNC_PIO    0x10000000 /* Driver should support PIO mode*/
>>
>>
>> In the bus driver,
>>
>> Inside master_xfer() callback we will check whether the intended i2c
>> transaction is meant to be processed in PIO mode, if yes, then we will
>> call PIO api, else as usual non-pio api should serve.
>>
>>
>> static int xxx_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[], int
>> num)
>> {
>>      ...
>>
>>
>> /* We can also check i2c_check_functionality(adap,I2C_FUNC_PIO) */
>>      if (msg->flags & I2C_M_PIO) {
>>          /* PIO mode operation */
>>      } else {
>>          /* non-pio mode of operation */
>>      }
>>
>>
>>      ...
>>
>>
>>          return ret;
>> }
>>
>>
>> Feedback/comments are always welcome.
>>
>> Thanks,
>> Vaibhav
>
>
> Any comments on the RFC?
>
> Also,
> + linux-arm & Wolfram Sang
>

Wolfram,

Any comment on this RFC?

Thanks,
Vaibhav